The problem of transmitting an extra information channel further on ordinary telephony channels has previously been solved in several ways:
(1) A service channel can be transmitted by integrating a copper pair in a fibre cable, for example. This is cheap and relatively reliable, but such a solution is not accepted by certain telephone administrations nor in certain markets.
(2) A service channel can be transmitted by using bits in the regular bit flow, e.g. by manipulating data as is described in Swedish patent application No. 8105914-9. In this case the transmission of the extra information channel is entirely digital, and coders/decoders or complicated synchronizing circuits are required to enable injection or extraction of the extra information bits.
(3) A further method of transmitting a service channel is described in "Electronic Letters" July 31, 1980, Vol. 16 No. 16 "Novel Supervisory Channel for Fiber Optic Transmission System".
(4) Finally, there is described in Philips Telecommunication Review, Vol. 40, No. 2, July 1982, "Philips Optical Fiber Transmission System: II. The Equipment Family", a method of transmitting an information channel by modulating the amplitude of the bit flow which is to transmit the regular data signals. The transmission of both service channel and data signals is performed with the aid of light pulses amplitude-modulated by the information channel.
According to item 2 above, it would in principle be possible to transmit a service channel by adding an extra telephone channel to the other channels transmitted by the regular bit flow. In this case, the extra channel must be coupled through all the repeaters in the connection to fill its function as a service channel, which causes problems by further demands having to be made on the transmission medium and the switching functions in the respective exchanges.